Polyethylene-bound catalysts examples

Polyethylene-supported catalysts that are initially insoluble but that become soluble on heating and are separated as insoluble materials on cooling are also used as catalysts in polymerization reactions. Infact, this was the first way a polyethylene-bound catalyst was used (Eq. 11) [34]. However, soluble polymers used in this manner appear to have several deficiencies. First, separation of the products from the catalyst may notalways be as simple as was the case with catalysts like 11 or 12 and low molecular weight products. For example, while a hot solution of a polyethylene-bound neodymium salt was successfully used in the stereoselective polymerization of butadiene to form high molecular weight (Z)-poly( 1,4-butadiene), the product mixture after cooling was a thick, viscous sus-... 

Non-cross-linked polymers can be used in this way just as cross-linked polymers can. For example, we have used polyethylene supports with surface grafts to support Pd(0) catalysts [133,134]. In these cases, the polymer-immobilized catalyst is used in exactly the same way as an insoluble polymer-bound catalyst. Such supported catalysts do require that the insoluble polymer be swollen or permeable to substrates or that the catalysts be within a solvent-permeable, thin immobilized graft. While this approach can be useful, it takes no advantage of the polymer s solubility. It is an approach that conceptually is no different than that used with insoluble inorganic supports or with polymers that are by design insoluble by virtue of cross-linking, and is an approach to catalyst immobilization that is not further discussed since this review is focused on polymer-immobilized catalysts that are used under solution-state conditions. 

We have successfully used this polyethylene-bound cluster catalyst to homogeneously catalyzed oxidation of both primary and secondary alcohols (equation 19), In the case of the primary alcohols, the oxidation stopped at the aldehyde stage. Representative examples of these oxidations are listed in Table 3. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Pd is an expensive metal. In Pd(0) or Pd(II)-catalyzed reactions, particularly in commercial processes, repeated uses of Pd catalysts are required. When products are low-boiling, they can be separated from the catalyst by distillation. The Wacker process for the production of acetaldehyde is an example, hi order to separate from less volatile products, there are several approaches for the economical use of Pd catalysts. Active Pd complexes covalently bound to a polymer chain are frequently used. After the reaction, the supported catalyst can be recovered by filtration and reused several times. Polymers such as the Merrifield resin [25], amphiphilic poly(ethylene glycol)-polystyrene copolymer [26] and polyethylene [27] are typical examples. Also polymer-supported microencapsulated Pd is used as a reusable... 

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